rac-Me2Si(2-Me-4-t-Bu-C5H2)2ZrMe+: An alkyl zirconocenium cation stabilized by steric shielding against interaction with ancillary ligands

The sterically shielded cation rac-Me2Si(2-Me-4-t-Bu-C5H2)2ZrMe+, generated by reaction of rac-Me2Si(2-Me-4-t-Bu-C5H2)2ZrMe2 with methylalumoxane (MAO) or with dimethylanilinium or trityl perfluorotetraphenylborate, differs from other, more open zirconocene methyl cations in that it does not form the symmetric AlMe3 adduct rac-Me2Si(2-Me-4-t-Bu-C5H2)2Zr(μ-Me)2AlMe2+. Instead, the unsymmetric ternary adduct rac-Me2Si(2-Me-4-t-Bu-C5H2)2ZrMe(μ-Me)AlMe3−NMe2Ph+ is formed in the presence of dimethylaniline and trimethylaluminum. UV/vis and 1H NMR characteristics of hydrocarbon solutions containing rac-Me2Si(2-Me-4-t-Bu-C5H2)2ZrMe+···B(C6F5)4- or rac-Me2Si(2-Me-4-t-Bu-C5H2)2ZrMe+···MAOMe- indicate that cation−anion association in these ion pairs is unusually weak.